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Basic knowledge of maintenance-free lead-acid batteries

- Nov 26, 2018-

Basic knowledge of maintenance-free lead-acid batteries

The regular name of the maintenance-free battery that people often say is called valve-regulated sealed lead-acid battery. The valve-regulated sealed lead-acid battery has a casing, a valve cover and a terminal block as seen from the outside. The sealing material around the terminals is red and black (or blue) to indicate the positive and negative electrodes. The 12V battery is divided into six independent isolated cells, each of which has a positive plate group and a negative plate group connected by respective bus conductors. The plate of the lead-acid battery is like a reinforced concrete structure. It is formed by coating (or rolling) an active material on the mesh-like skeleton of the alloy wire: the material on the positive electrode plate is lead dioxide (PbO2), and the negative electrode plate The substance on it is velvet lead (Pb). Each of the positive and negative plates is interspersed with a porous microfiber material (also filled with silica gel material), in which a sulfuric acid (H2SO4) electrolyte is adsorbed, and the fiber material (or silica gel material) is electrochemically reacted. The process of liquid phase transport and gas phase transport in the process, which is tightly assembled with the positive and negative plate groups to form a 2V battery cell. Since lead-acid batteries inevitably generate hydrogen and oxygen during charging, they generate pressure in the cells when they are too much and too late to form and form water. In order to ensure the normal and safe operation of the battery, each cell has its own overflow valve, which allows the gas to escape automatically when the pressure is excessive. Compared with the rich liquid battery filled with the electrolyte body in the battery tank, the valve-regulated sealed lead-acid battery contains only a small amount of electrolyte inside, which is a poor liquid battery. However, due to the certain redundancy of the electrolyte involved and the reasonable use of the overflow valve pressure, the water loss caused by the escape of the gas is extremely small, so that the electrolyte of the valve-controlled battery is basically in the life process. No need to replenish, so valve-regulated sealed lead-acid batteries have also become maintenance-free batteries.

How much is the voltage of the battery normal?

It is often said that this battery voltage is 12V. The 12v mentioned here refers to the most basic parameter of the battery - the nominal potential (unit v). A lead-acid battery has a nominal potential of 2v, and the nominal potential of six single-string batteries is 12v. The power supply used in electric vehicles is generally composed of 2 to 5 12v batteries connected in series to form 24v, 36v, 48v, 60v battery packs. Here, the theoretical values determined by the characteristics of the active materials used in the batteries are determined. In fact, there are differences in the voltage and nominal potential of the battery under different conditions. For example, a normal lead-acid battery with a nominal potential of 12v is at the end of the charging process, the charging polarization reaches a maximum value, and the voltage can reach 14.4v or higher; at the end of the discharge, the discharge polarization reaches a maximum value. The voltage can be as low as 9v. After charging or discharging is stopped and left for a few hours, the polarization voltage (concentration plan) completely disappears. The potential of this 12v battery can be between 13.8v (after filling) and 11v (after discharging). It is caused by a change in the state of the active material inside the battery.

What is the meaning of battery capacity (Ah)?

The rated capacity of the battery, c, is the product of the discharge current (A) and the discharge time (h). Since Ah obtained by using different discharge parameters for the same battery is different, in order to facilitate description, measurement, and comparison of battery capacity, uniform conditions must be set in advance. In practice, battery capacity is defined as the amount of electricity given by the set current to discharge the battery to the set voltage. It can also be said that the battery capacity is the product of the time elapsed between discharging the battery to the set voltage with the set current and the current. In order to set uniform conditions, firstly, according to the difference in battery construction characteristics and use, several discharge time rates are set. The most common ones are 20 hours and 10 hours, and the electric vehicle battery is 2 hours. It is written as C20. C10 and C2, where C represents the battery capacity, followed by a number indicating the number of hours that the battery is discharged to a set voltage with a current of a certain intensity. Thus, the rated discharge current is obtained by dividing the number of hours by the capacity. That is to say, batteries with the same capacity and different discharge rates have a far different nominal discharge current. For example, an electric bicycle has a battery capacity of 10 Ah and a discharge rate of 2 hours. It is written as 10 Ah2, and its rated discharge current is 10 (Ah) / 2 (h) = 5 A; and a battery for starting a car has a capacity of 54 Ah. The discharge rate is 20 hours, written as 54Ah20, its rated discharge current is only 54 (Ah) / 20 (h) = 2.7A! In other words, if the two batteries are discharged with 5A and 2.7A respectively , it should last for 2 hours and 20 hours to fall to the set voltage. The above-mentioned set voltage refers to the termination voltage (unit V). The termination voltage can be simply understood as: the battery voltage drops during discharge to a minimum value that does not cause damage. The termination voltage value is not fixed. It decreases as the discharge current increases. The larger the discharge current of the same battery, the lower the termination voltage can be, and vice versa. That is to say, when the large current is discharged, the battery voltage is allowed to drop to a lower value, and a small current discharge is not possible, otherwise damage may be caused. The current intensity of the battery during operation is also often expressed in terms of magnification, written as NCh. N is a multiple, C is the number of hours of capacity, and h is the number of hours specified by the rate of discharge. Here, the value of h is only used as a reminder that the relevant battery belongs to this discharge time rate, so the battery that specifically describes a certain time rate is that the magnification is often written in the form of NC without writing down the standard. Multiplying the multiple N by the capacity C is equal to the current A. For example, 20Ah uses 0.5c rate discharge, 0.5×20=10A. For another angle example: a car starting battery capacity 54Ah, measured output current is 5.4A, then his discharge rate N is 5.4 / 54 = 0.1C.

How lead-acid batteries work

1. Lead-acid battery electromotive force generation

After the lead-acid battery is charged, the positive electrode lead dioxide (PB02), the action of water molecules in the sulfuric acid solution, a small amount of lead dioxide and water to form a dissociable unstable substance - lead hydroxide (Pb(OH)4) The hydroxide ion is in the solution, and the lead ion (Pb4) remains on the positive electrode plate, so the electrons are absent on the positive electrode plate. After the lead-acid battery is charged, the negative electrode plate is lead (Pb), and the sulfuric acid in the electrolyte (H2S04) The reaction turns into lead ions (Pb2), and the lead ions are transferred to the electrolyte, leaving two electrons (2e) remaining on the negative plate. Courseware, when the external circuit is not connected (battery open circuit), due to chemical action, there is no electron on the motor board, and the negative plate has more electrons, and a certain potential difference is generated between the two plates. This is the electromotive force of the battery.

2. Electrochemical reaction of lead-acid battery during discharge

The lead-acid battery is placed on the TV, and the potential difference of the battery acts on the cathode. The electrons on the negative plate enter the positive plate through the load to form a current. At the same time, a chemical reaction is formed inside the battery. After two electrons are emitted from each lead atom on the negative electrode plate, the generated lead ions (Pb2) react with the sulfate ion (S04-2) in the electrolyte to form insoluble lead sulfate (PbS04) on the electrode plate. The oxygen ions (0-2) hydrolyzed by the positive electrode plate react with hydrogen ions (H) in the electrolytic solution to form a stable substance water. Under the action of the electric field of the sulfate ion and the hydrogen ion existing in the electrolyte, the positive and negative electrodes of the battery are respectively moved to form a current inside the battery, and the entire circuit is formed, and the battery is continuously discharged outward. The concentration of H2S04 decreases continuously during discharge, the lead sulfate (PbS04) on the positive and negative electrodes increases, the internal resistance of the battery increases (the lead sulfate does not conduct electricity), the electrolyte concentration decreases, and the battery electromotive force decreases.

3. Electrochemical reaction of lead-acid battery charging process

When charging, the external power source (charging pole or rectifier) should be connected externally to restore the material generated by the positive and negative plates after discharge to the original active material, and convert the external electrical energy into chemical energy for storage. On the positive electrode plate, lead sulfate is dissociated into divalent lead ions (Pb2) and sulfate negative ions (SO4-2) under the action of external current. Since the external power source continuously draws electrons from the positive electrode, the second of the positive electrode plates is colored. The valence lead ion Pb2) continuously releases two electrons to be added, becomes tetravalent lead ions (Pb4), and continues to react with water, eventually producing lead dioxide (PbO2) on the positive electrode plate. On the negative electrode plate, lead sulfate is dissociated into divalent lead ions (Pb2) and sulfate negative ions (SO4-2) under the action of external current. Since the negative electrode continuously obtains electrons from the external power source, the free electrode near the negative electrode plate The valence lead ion (Pb2) is neutralized to lead (Pb) and adheres to the negative electrode plate as velvet lead. In the electrolyte, the positive electrode continuously generates free hydrogen ions (H) and sulfate ions (SO4-2), and the negative electrode continuously generates sulfate ions (SO4-2). Under the action of the electric field, hydrogen ions move to the negative electrode, and sulfate is generated. The ions move toward the positive electrode to form a current. At the end of charging, under the action of external current, electrolytic reaction of water will occur in the solution.

4。 Change of electrolyte after charging and discharging of lead-acid battery

It can be seen from the above that when the lead-acid battery is discharged, the sulfuric acid in the electrolyte is continuously reduced, the water is gradually increased, and the specific gravity of the solution is decreased. It can be seen from the above that when the lead-acid battery is charged, the sulfuric acid in the electrolyte is continuously increased, the water is gradually decreased, and the specific gravity of the solution is increased. In actual work, the charge of the lead-acid battery can be judged according to the change of the specific gravity of the electrolyte. Use and maintenance of maintenance-free lead-acid batteries In recent years, with the deepening of the two-grid transformation of power systems, high-frequency switching power supplies and maintenance-free lead-acid batteries manufactured using switching power supply technology have been widely used. However, due to insufficient operating experience, the maintenance of the DC power supply, especially the battery, is not in place, so that the reliability of the DC power supply cannot be effectively guaranteed.

Meaning of maintenance free battery

The main advantage of the valve-regulated lead-acid battery is that the oxygen generated on the positive electrode plate during charging is reduced to water on the negative electrode plate by recombination reaction, and it is not necessary to add water for maintenance during the specified floating charge life. Maintain lead-acid batteries. It can be seen that the maintenance-free operation is only compared with the ordinary battery, and the project of adding pure water or distilled water to adjust the electrolyte liquid level is omitted during the operation, and it is not necessary to remove all maintenance work.


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